1 MICROSTRUCTURE DEVELOPMENT IN HYPEREUTECTIC CAST Al-Si ALLOYS EVALUATED BY METALLURGICAL ANALYSIS AND NEUTRON DIFFRACTION W. Kasprzak 1 , D. Sediako 2 , M. Aniolek 1 , H. Kurita 3 1 Natural Resources Canada, CanmetMATERIALS, 183 Longwood Ave. South, Hamilton, Ontario, Canada 2 National Research Council, Canadian Neutron Beam Centre, Chalk River, Ontario, Canada 3 R&D Operations, Yamaha Motor Co. Ltd. 2500 Shingai, Iwata, Shizuoka, 438-8501, Japan Keywords: Hypereutectic Al-19%Si alloys, alloying with Cu, Mg, P, microstructure, in-situ neutron diffraction, thermal analysis, hardness, tensile strength. Abstract Presented results show individual effect of key alloying elements, i.e., 2.8%Cu, 0.7%Mg and 0.01%P on the as-cast microstructure development in the hypereutectic Al-19%Si alloy evaluated using classical metallurgical analysis as well as in-situ neutron diffraction during alloy solidification process. Neutron diffraction revealed possible Si atoms clustering above liquidus temperature i.e., 677 ºC and pre-mature nu -Al crystallites below liquidus temperature i.e., 667 and 625 ºC in addition to liquid-to-solid phase transformation assessment during solidification. Mechanical strength i.e., hardness and ultimate tensile strength improvement due to Cu, Mg and P additions is evidenced and linked with microstructure evolution under non-equilibrium solidification conditions. Primary Si refinement was improved with subsequent addition of Cu and Mg, and P addition alone had insignificant effect on primary Si refinement. Introduction Rising demands to reduce the gasoline consumption without compromising vehicle performance and overall safety standards requires development of a high strength-low weight materials and their cost-effective manufacturing processing technologies. Such activities became high priority for vehicles powered by Internal Combustion (IC) engines as well as hybrid gasoline-electric and electric powertrain since overall weight reduction ensures the increase in driving range by compensating additional vehicle weight caused by the battery pack. Hypereutectic Al-Si alloys containing up to 16-20% Si are extensively used in automotive industry for applications where high operating temperature and trybological performance are required [1-6]. In addition to light weight, a good thermal conductivity of such alloy is well known and utilized for rapid combustion heat removal in internal combustion engines. Exemplary components include monolitic engine blocks cast using High Pressure Die Casting (HPDC) process, cylinder bore inserts for sand cast engine blocks, engine pistons made using Low Pressure Permanent Mold (LPPM), HPDC or hot forging processes. Microstructure control during component manufacturing is important and includes aspects of primary and eutectic Si modification as well as development of primary Si-free composite-type materials with improved fatigue performance required for cyclic-type loading powertrain applications [1,3,4]. 13 th International Conference on Aluminum Alloys (ICAA13) Edited by: Hasso Weiland, Anthony D. Rollett, William A. Cassada TMS (The Minerals, Metals & Materials Society), 2012 1431